Rapid burial and intense degradation of organic matter drive active silicate weathering in the subsurface sediments of the ocean’s deepest realm

Hadal trenches (>6000 m water depth) have been revealed as hotspots of organic carbon burial and microbial respiration in the ultradeep ocean environment. However, understanding of the anaerobic metabolic pathways and rates, as well as carbon-silicon cycling, in the hadal trench sediments remains very fragmentary because of the shallow nature of traditional coring penetration. Using materials collected during International Ocean Discovery Program (IODP) Expedition 386 in the Japan Trench and a reaction-transport model, we provide a regional quantitative assessment of organic carbon turnover by sulfate reduction, anaerobic oxidation of methane, and methanogenesis in addition to silicate weathering and authigenic carbonate and clay formation. We show that rapid burial of relatively labile organic carbon resulting from subduction earthquakes triggers organic carbon and methane turnover at rates comparable to those in continental margin sediments, thereby stimulating active silicate weathering and authigenic carbonate formation. Despite vigorous organic carbon turnover, the vast majority of organic carbon is buried, implying an important role of tectonic-associated events in translocating and preserving organic carbon in the deepest part of the ocean. These results quantitatively demonstrate, for the first time, active coupled carbon-silicon cycling in hadal trench sediments and have implications for the subduction zone carbon budget.